In the production of mass air flow sensors, it is required to adjust the electrical power to the mass air flow sensor's heater so that it will operate at a specified temperature and to adjust the mass air flow sensor's response time to a step change in the mass air flow rate being detected by the mass air flow sensor. Both of these adjustments may be made at the same calibration station if the calibration station can be provided with two different calibrated mass air flow rates. In the use of these mass air flow sensors, the operating characteristic of the manufactured mass air flow sensor must be uniform within specified limits in order to avoid subsequent trimming or adjustments of either the output signal of the mass air flow sensor or the electronic fuel control of an internal combustion engine.
In order to achieve these objectives, the mass air flow calibration must be accurate and repeatable. The use of a critical flow or sonic nozzle to produce a calibrated mass air flow rate is well known in the art. Hicks et al in U.S. Pat. No. 4,590,790 discloses the equation for determining the mass air flow rate through a critical flow or sonic nozzle used to calibrate a volumetric air flow meter on a high pressure pipe line. Evans et al in U.S. Pat. No. 3,226,973 discloses a flowmeter calibration stand using a sonic nozzle to provide a calibrated air flow from a pressurized air source to the flowmeter being calibrated. A pressure regulator is used to maintain the pressure across the sonic nozzle at a fixed value.
The prior art does not recognize the need for production calibration of mass air flow sensors so that their operating parameters are substantially identical, nor the requirement for different calibrated air flow rates for trimming or adjusting different operating parameters of the mass air flow sensors during production.